1. Field of the Invention
The present invention relates to OAM processing devices to support operations, administration, and maintenance (OAM) functions, and more particularly, to an OAM processing device used to manage the operations and maintenance of Asynchronous Transfer Mode (ATM) networks, being disposed at each ATM network element on local loops that interconnects subscriber premises and a switching system.
2. Description of the Related Art
ATM networks have expanded to cover a wide variety of communication services in recent years, enabling different types of communication traffic to be handled in an integrated way. This diversity in communication services has triggered a demand for more efficient OAM processing at ATM layer that facilitates ATM network management. Miniaturization and power reduction of OAM processing devices are among the key aspects of this demand.
FIG. 10 is a block diagram that shows a part of a conventional OAM processing device. This OAM processing device is designed for use in a network element deployed as part of local loops which connect subscriber premises and a switching system. In FIG. 10, an error detector 101 monitors incoming signals from the switching system to detect errors in communication channels, which signals are either synchronous frames based on the Synchronous Optical Network (SONET) standards or a stream of ATM cells. More specifically, the error detector 101 checks the overhead field of each synchronous frame and generates physical layer alarms when it detected any errors indicated therein, while watching the ATM cell stream to find a Virtual Path Alarm Indication Signal (VP-AIS) cell. When such an error or alarm signal is detected, a registered VPI/VCI generator 102 outputs a plurality of Virtual Path Identifiers (VPIs) and Virtual Channel Identifiers (VCIs) that have been registered beforehand. That is, a list of all VPIs/VCIs that correspond to a plurality of subscriber premises connected to the network element have been previously recorded in the registered VPI/VCI generator 102. When a physical layer alarm is indicated, the registered VPI/VCI generator 102 outputs the VPIs of all virtual paths (VPs) to be affected by the alarm. Similarly, when a VP-AIS cell is detected, the registered VPI/VCI generator 102 supplies the VPIs/VCIs of all virtual channels (VCs) that belong to a VP indicated in the detected VP-AIS cell.
When a physical layer alarm is detected, an OAM cell generator 103 creates OAM cells for VP-AIS (i.e., VP-AIS cell), using the VPIs received from the registered VPI/VCI generator 102, to alert the other network elements of the failure of all VPs related to the physical layer alarm. When in turn a VP-AIS cell is detected, the OAM cell generator 103 creates another kind of OAM cells, or Virtual Channel Alarm Indication Signal (VC-AIS) cells, based on the VPIs received from the registered VPI/VCI generator 102, to signify the failure of all VCs that belong to the failed VP.
An OAM cell switch 104 inserts the OAM cells created by the OAM cell generator 103 into the ATM cell stream. As for the OAM cells in the case of physical layer alarms, the OAM cell switch 104 can simply insert them into the traffic since there are no other valid cells flowing as a cell stream. In the case of VP-AIS cells, however, the OAM cell switch 104 should find idle cells in the cell stream and put the OEM cells in place of the idle cells found.
Separately from the above-described path, each ATM cell is subjected to a VPI/VCI extractor 105 to extract its VPI/VCI fields for verification in an incoming VPI/VCI verifier 106. The incoming VPI/VCI verifier 106 has the records of VPIs and VCIs corresponding to the subscriber premises connected to the network element, just as the registered VPI/VCI generator 102 has. The incoming VPI/VCI verifier 106 verifies each VPI/VCI extracted by the VPI/VCI extractor 105 by comparing it with the records registered therein. If there is such a VPI/VCI record that coincides with the extracted VPI/VCI, the incoming VPI/VCI verifier 106 recognizes the ATM cell as being addressed to one of the subscriber premises connected to the network element, and it will attach a single-bit flag to the 53-byte ATM cell to indicate the coincidence of VPI/VCI.
As described above, the conventional OAM processing devices are equipped with a memory in both the registered VPI/VCI generator 102 and incoming VPI/VCI verifier 106 to store VPIs/VCIs corresponding to the subscriber premises connected to the present network element. The VPI/VCI bit length is 24-bit maximum, and the number of VPs and VCs actually increases as the network element has more physical links. To support this full range of VPIs/VCIs, the registered VPI/VCI generator 102 and incoming VPI/VCI verifier 106 must have a large amount of VPI/VCI storage. This VPI/VCI storage requirement leads to an increased scale of memory circuits, which makes the OEM processing device larger and more power consuming. It is therefore desirable to unify the functions of the registered VPI/VCI generator 102 and incoming VPI/VCI verifier 106 so that they can share a common set of VPI/VCI records.
Meanwhile, to perform ATM switching at the network element, some VPI/VCI conversion tables to translate the VPI/VCI of each ATM cell to new one and means for replacing its header information must be disposed immediately next to the registered VPI/VCI generator 102 and to the incoming VPI/VCI verifier 106. Since this duplexity of VPI/VCI conversion tables will increase the scale of the control circuit, it is also desirable to develop some techniques to unify those functions.
The OAM processing device comprises three major functional units as: alarm handler unit, performance monitor (PM) unit, and usage parameter control (UPC) unit. In conventional OAM processing devices, those units have their own circuit block to extract and verify the VPI/VCI of incoming ATM cells. Furthermore, they individually check the range of the VPI/VCI of each incoming ATM cell based on a predetermined range definition. Such functional duplexity is another factor to increase the scale of the control circuit.
The PM unit provides performance monitoring and measurement functions, where a large amount of measurement data should be collected at predetermined PM block intervals (i.e., time slots for measurement defined by two consecutive PM cells) separately for each VP/VC channel. Actually, the conventional OAM processing devices have a large memory to store the measurement data and a processor to summarize the measurement data for individual VP/VC channels by summing up the data for all PM blocks. This memory requirement makes it difficult to reduce the circuit size and power consumption. Further, since the heavy calculation load is imposed on the processor, it is unable to support enough VP/VC channels.